Two little girls in blue - Mary Higgins Clark

Margaret and Steve Frawley celebrate the third birthday of their twin girls, Kelly and Kathy, with an afternoon party in their new home. That evening, Steve and Margaret attend a black-tie dinner in New York. On returning home, the police are in the house, the babysitter has been found unconscious, the children are gone and a note demanding an eight million dollar ransom has been left in their room.
Steve firm agrees to pay the ransom. The kidnapper, who identifies himself as the "Pied Piper", makes his terms known - on delivery of the ransom, a call will come revealing the girls' whereabouts. The call comes, but only Kelly is in the car parked behind a deserted restaurant. The driver is dead from a gunshot wound and has left a suicide note, saying he had inadvertently killed Kathy and had dumped her body in the ocean.
At the private memorial Mass for Kathy, Kelly tugs Margaret's arm and says: "Mommy, Kathy is very scared of that lady. She wants to come home right now." More unexplainable occurrences follow, indicating that Kelly is in touch with Kathy. At first, no one except the mother believes that the twins are communicating and that Kelly is still alive. As Kelly's warnings become increasingly specific and alarming, however, the FBI agents set out on a search for Kathy. As they close in on the Pied Piper and his accomplices, Kathy's life hangs on a thread.